For automatic assembly of a workpiece which is carried in a working line, there has been known a method for positioning the workpiece at a predetermined position on a workpiece carrying pallet by means of providing a positioning pin on the pallet and then fitting a positioning hole of the workpiece to the positioning pin. Such a prior method is disclosed in, for example, the Japanese Utility Model Publication No. 59-120685. In case of different kinds of workpieces which have different configurations carried on the same working line, the position of the positioning pin on the pallet has to be changed or relocated in accordance with each workpiece. Therefore, it is necessary to prepare a plurality of different kinds of pallets which have the position pin arranging at a variety of suitable locations each designed to support a different workpiece and to select one of these pallets in accordance with the particular workpiece passing along the working or production line. As an alternative, one could provide a plural of detachable pins on the pallet and to repeat the steps of setting and removing the pins according to each workpiece passing along the working or production or mass assembly line. In either case, changing the pallets or the pins according to each workpiece passing along the working line is troublesome and to be avoided if possible.